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On a recent project for a outdoor shed the roof support structure did not look like one of the conventional roof truss designs I have encountered. See snapshot below. (The span is 10 m with the support members 250x125 mm beams)

enter image description here

Is this still a truss? Or what is this sort of support framework called?

It looks like a textbook King Post Truss (to me) but without the vertical member.

enter image description here

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This looks like a Collar Beam Roof (or sometimes a Collar Tie Roof). It is often used in lightly-loaded residential construction. One popular use is to form Cathedral Ceilings.

Strictly speaking this is not a truss since the structure relies primarily on bending of the rafters to support the roof loads. By definition trusses are assumed to consist entirely of tension/compression elements and bending of these elements is minimal.

In this case the higher up the collar beam is the more bending would be present in the rafters which can result in some outwards movement of the walls. This is not likely to be an issue in a shed but could lead to some drywall cracking in a finished interior.

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  • $\begingroup$ Thanks! Is there a standard relative height where the collar beam is placed? The engineering drawing I have gives details of all other dimensions but the only one I don't see is the length / relative height spec. for that horizontal member in the Collar Beam. Maybe the designer assumed a standard setup? Is the exact position of the horizontal member relevant to the design? $\endgroup$ – curious_cat May 1 '16 at 11:55
  • $\begingroup$ For costing the design I've assumed the Collar Beam length as half the total span (eyeballing the drawing). Is that a good assumption? $\endgroup$ – curious_cat May 1 '16 at 12:41
  • $\begingroup$ I'm not aware of any standard for the height of the collar beam. The position of the horizontal member will have an effect on the size of the rafters - the higher up the more bending in the rafters -> larger rafters. I don't know if that is a good assumption for costing but the collar beam wont be longer than the distance between the walls - so that at least will give you a maximum. $\endgroup$ – atom44 May 2 '16 at 21:36

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